ulTaskNotifyTake() for binary/counting notification in FreeRTOS - Mini Project: Build & Apply

Using ulTaskNotifyTake() for Binary and Counting Notifications in FreeRTOS

📖 Scenario: You are working on a FreeRTOS-based embedded system where tasks need to wait for events signaled by other tasks or interrupts. You will learn how to use ulTaskNotifyTake() to wait for binary and counting notifications.

🎯 Goal: Build a simple FreeRTOS program where a task waits for notifications using ulTaskNotifyTake(). You will set up a task, configure notification values, and handle both binary and counting notifications.

📋 What You'll Learn

Create a task handle variable called xTaskHandle

Create a variable called ulNotificationValue to store the notification count

Use ulTaskNotifyTake(pdTRUE, portMAX_DELAY) to wait for a notification and clear it

Print the notification count stored in ulNotificationValue

💡 Why This Matters

🌍 Real World

In embedded systems, tasks often need to wait for events like sensor signals or communication messages. Using ulTaskNotifyTake() helps tasks wait efficiently for these events without wasting CPU time.

💼 Career

Understanding task notifications is essential for embedded software developers working with FreeRTOS to build responsive and efficient real-time applications.

1

Create a task handle variable

Create a task handle variable called xTaskHandle and initialize it to NULL.

FreeRTOS

# Create a task handle variable called xTaskHandle and set it to NULL
# Your code here

Hint

Use the type TaskHandle_t and set xTaskHandle to NULL.

2

Create a variable to store notification count

Create a variable called ulNotificationValue of type uint32_t to store the notification count.

FreeRTOS

TaskHandle_t xTaskHandle = NULL;
# Create a uint32_t variable called ulNotificationValue
# Your code here

Hint

Use uint32_t type and initialize ulNotificationValue to 0.

3

Use ulTaskNotifyTake() to wait for notification

Use ulTaskNotifyTake(pdTRUE, portMAX_DELAY) to wait for a notification and clear it. Store the return value in ulNotificationValue.

FreeRTOS

TaskHandle_t xTaskHandle = NULL;
uint32_t ulNotificationValue = 0;
# Use ulTaskNotifyTake(pdTRUE, portMAX_DELAY) and store result in ulNotificationValue
# Your code here

Hint

Call ulTaskNotifyTake() with pdTRUE to clear notification on exit and portMAX_DELAY to wait indefinitely.

4

Print the notification count

Print the value of ulNotificationValue using printf to show how many notifications were received.

FreeRTOS

TaskHandle_t xTaskHandle = NULL;
uint32_t ulNotificationValue = 0;
ulNotificationValue = ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
# Print ulNotificationValue
# Your code here

Hint

Use printf("Notification count: %lu\n", ulNotificationValue); to print the count.

Practice

1. What does the ulTaskNotifyTake() function do in FreeRTOS?

easy

A. It deletes a task from the scheduler.

B. It creates a new task in the system.

C. It sends a notification to another task.

D. It blocks a task until a notification is received.

Solution

1. Step 1: Understand the purpose of ulTaskNotifyTake()

   This function is designed to make a task wait (block) until it receives a notification from another task or interrupt.

2. Step 2: Differentiate from other task functions

   Creating, sending, or deleting tasks are done by other FreeRTOS functions, not ulTaskNotifyTake().

3. Final Answer:

   It blocks a task until a notification is received. -> Option D

4. Quick Check:

   ulTaskNotifyTake() waits for notification [OK]

Hint: ulTaskNotifyTake() makes task wait for notification [OK]

Common Mistakes:

• Confusing ulTaskNotifyTake() with task creation functions
• Thinking it sends notifications instead of receiving
• Assuming it deletes tasks

2. Which parameter of ulTaskNotifyTake() controls whether the notification count resets on exit?

easy

A. xClearCountOnExit

B. xTicksToWait

C. ulNotificationValue

D. xNotifyOption

Solution

1. Step 1: Identify parameters of ulTaskNotifyTake()

   ulTaskNotifyTake() takes two parameters: xClearCountOnExit and xTicksToWait.

2. Step 2: Understand xClearCountOnExit role

   This parameter decides if the notification count resets to zero when the function exits (pdTRUE) or not (pdFALSE).

3. Final Answer:

   xClearCountOnExit -> Option A

4. Quick Check:

   xClearCountOnExit resets count [OK]

Hint: xClearCountOnExit resets notification count [OK]

Common Mistakes:

• Mixing xClearCountOnExit with xTicksToWait
• Assuming ulNotificationValue is a parameter
• Confusing with notification sending options

3. Consider this code snippet:

uint32_t count = ulTaskNotifyTake(pdFALSE, portMAX_DELAY);
// Assume 3 notifications were sent before this call

What will be the value of count after this call?

medium

A. 1

B. 0

C. 3

D. portMAX_DELAY

Solution

1. Step 1: Analyze xClearCountOnExit parameter

   pdFALSE means the notification count is not cleared on exit, so it counts all notifications received.

2. Step 2: Understand the count returned

   Since 3 notifications were sent before, ulTaskNotifyTake() returns the total count, which is 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   pdFALSE keeps count, so count = 3 [OK]

Hint: pdFALSE keeps count, so returns total notifications [OK]

Common Mistakes:

• Assuming count resets to 1 regardless
• Confusing portMAX_DELAY as return value
• Thinking count is zero if notifications sent before

4. What is wrong with this code snippet?

uint32_t count = ulTaskNotifyTake(pdTRUE, 100);
if(count == 0) {
    // No notification received
}
// Task expects to count multiple notifications

medium

A. Using pdTRUE clears count, so multiple notifications can't be counted.

B. Timeout value 100 is invalid and causes error.

C. ulTaskNotifyTake() cannot be used with if statements.

D. count variable should be int, not uint32_t.

Solution

1. Step 1: Check xClearCountOnExit usage

   pdTRUE clears the notification count on exit, so only a binary notification is counted, not multiple.

2. Step 2: Understand task expectation

   The task expects to count multiple notifications, but pdTRUE resets count, so it cannot accumulate multiple counts.

3. Final Answer:

   Using pdTRUE clears count, so multiple notifications can't be counted. -> Option A

4. Quick Check:

   pdTRUE clears count, no multiple counting [OK]

Hint: pdTRUE clears count; use pdFALSE for counting multiple [OK]

Common Mistakes:

• Thinking timeout 100 is invalid
• Believing ulTaskNotifyTake() can't be in if
• Confusing variable type for count

5. You want a task to sleep until it receives exactly 2 notifications, then proceed. Which ulTaskNotifyTake() usage is correct?

hard

A. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) twice and sum results.

B. Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2.

C. Call ulTaskNotifyTake(pdTRUE, 0) in a loop until count is 2.

D. Call ulTaskNotifyTake(pdTRUE, portMAX_DELAY) once and check if return is 2.

Solution

1. Step 1: Understand counting notifications

   Using pdFALSE keeps the count of notifications, so the function returns the total number received.

2. Step 2: Use single call to wait indefinitely

   Calling ulTaskNotifyTake(pdFALSE, portMAX_DELAY) waits until at least one notification arrives and returns the count accumulated.

3. Step 3: Check if count equals 2

   After the call returns, check if the count is exactly 2 to proceed.

4. Final Answer:

   Call ulTaskNotifyTake(pdFALSE, portMAX_DELAY) once and check if return is 2. -> Option B

5. Quick Check:

   pdFALSE counts notifications; check return for 2 [OK]

Hint: Use pdFALSE and check return count for exact notifications [OK]

Common Mistakes:

• Calling ulTaskNotifyTake multiple times and summing counts
• Using pdTRUE which resets count
• Using zero timeout causing immediate return